Problem: Solve for $z$ : $27 + z = 9$
Explanation: Subtract $27$ from both sides: $ \begin{eqnarray} \\ 27 + z &=& 9 \\ \\ {-27} && {-27} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{27 + z} &\hphantom{=}& \hphantom{9} \\ z &=& 9 {- 27} \\ \end{eqnarray} $ Simplify. $ z = -18$